The Best Insulation Boards for a New Build

Choosing the right insulation boards for a new build is essential for achieving long‑term thermal performance, reducing heat loss, and meeting increasingly strict Building Regulations Part L. With energy targets tightening across the UK, rigid insulation boards – particularly PIR insulation and Kingspan Kooltherm phenolic insulation – have become the preferred choice for architects, contractors and developers.

Below, we break down the benefits of both PIR and Kooltherm, and highlight the best options for pitched roofs, flat roofs, floors, and cavity walls.

Why PIR Insulation Is Ideal for New Builds

PIR (polyisocyanurate) insulation is one of the most commonly used materials in modern housebuilding thanks to its strong performance‑to‑thickness ratio and ease of installation.

• Excellent thermal performance with low lambda values
• Lightweight, rigid boards that are easy to cut and handle
• Foil facings that assist with vapour control
• Large sheet sizes help reduce installation time
• Suitable for floors, roofs, walls and cavity applications
• Available in a wide range of thicknesses (25 mm – 150 mm)

PIR is especially popular for developers working across multiple plots where consistency, affordability and insulation efficiency are priorities.

Kingspan Kooltherm Is Used in Premium Builds

Kooltherm is Kingspan’s premium phenolic insulation range. It delivers significantly better thermal conductivity than PIR, allowing builders to meet strict U-values using much slimmer build‑ups.

• Best-in-class thermal performance
• Ideal where space is limited
• Meets very low U-values with minimal thickness
• Perfect for energy‑efficient new builds

Although Kooltherm is more expensive per board, the reduced thickness often saves time, reduces structural adjustments, and improves overall building performance.

Best Insulation Boards by Application

Here is a breakdown of the ideal products for each major component of a new‑build home. Where appropriate, links to our product ranges are included.

1. Pitched Roof Insulation

Best Choice: Kingspan Kooltherm K107 Pitched Roof Board

K107 offers exceptional thermal efficiency for pitched roofs, particularly where rafter depth is limited.

• Achieves low U-values without requiring deeper rafters
• Lightweight and easy to cut around roof features
• Ideal for warm roof and hybrid pitched roof systems

2. Flat Roof Insulation

Best Choice: Kingspan TR Range (TR27, TR26, TR24)

Kingspan’s TR range is designed specifically for use within warm flat roof systems and is engineered to provide durability, compressive strength and moisture resistance.

• Compatible with single‑ply membranes, felt systems and liquid roofing
• High compressive strength for both trafficked and non‑trafficked roofs
• Moisture‑resistant foil facings

3. Floor Insulation

Best Choice: Kingspan Kooltherm K103 Floorboard

K103 provides outstanding thermal conductivity, making it ideal for achieving low U-values in new-build floors without increasing build‑up thickness.

• Suitable for concrete slabs, screed floors, floating floors and beam‑and‑block
• Reduces cold bridging at floor junctions
• Premium performance for energy‑efficient homes

4. Cavity Wall Insulation

Best Choices:

• Kingspan K106 Full Fill Cavity Board
• Kingspan K108 Partial Fill Cavity Board

Kingspan K106 – Full Fill

K106 is engineered for full‑fill cavity wall construction, providing extremely low thermal conductivity while maintaining moisture control.

• Ideal for slim cavity designs
• Moisture‑resistant facings
• Improves airtightness and reduces thermal bypass

Kingspan K108 – Partial Fill

K108 is designed for partial‑fill cavities, maintaining a ventilated gap while still offering premium thermal performance.

• Phenolic core for excellent thermal resistance
• Ensures drainage and moisture management
• Widely used in masonry construction

PIR or Kooltherm — Which Should You Use?

Choosing the Best Insulation for a New Build

A fabric‑first approach is the key to building comfortable, energy‑efficient homes that stand the test of time. PIR boards offer excellent performance at a cost‑effective price and suit the majority of new‑build applications. Kingspan Kooltherm takes thermal efficiency a step further, helping you achieve very low U‑values with reduced thickness.

By combining the right boards — such as K107 for pitched roofs, the TR range for flat roofs, K103 for floors, and K106 or K108 for cavity walls — developers can meet regulatory requirements while delivering warm, efficient and future‑proof homes.